Filtration cartridges are well known devices used in many applications to separate substances such as particles, microorganisms, dissolved species, etc. from their carrier fluid. These cartridges are formed of one or more filtration membranes, either in the form of a flat sheet or in the form of hollow fibers, which are secured within a housing. Cartridges are configured so that the fluid to be filtered enters through an inlet, passes through the membrane filter, and the filtered fluid exits through an outlet. In some configurations, a portion of the entering fluid is removed through a second outlet as a concentrated stream. The membrane(s) provide a semi-permeable barrier that separates the inlet from the outlet so as to achieve filtration.
Filter cartridges are comprised of a membrane filter, a housing in which the filter is located and fluid-tight seals. Membrane filters are porous structures having average pore sizes of from about 0.005 micron to about 10 micron. Membranes with average pore size of from about 0.002 to about 0.05 micron are generally classified as ultrafiltration membranes. Ultrafiltration membranes are used to separate proteins and other macromolecules from aqueous solutions. Ultrafiltration membranes are usually rated in terms of the size of the solute they will retain. Typically, ultrafiltration membranes can be produced to retain dissolved or dispersed solutes of from about 1000 Daltons to about 1,000,000 Daltons. They can be rated by Molecular Weight Cutoff, which is the molecular weight expressed in Daltons, a unit of molecular mass, at which a stated percent of the feed concentration of the solute being processed is retained or rejected by the membrane. Manufacturers usually set the stated percent at 90% to 95%. Membranes with pore sizes of from about 0.05 to 10 microns are generally classified as microporous membranes. Microporous membranes are used in a wide variety of applications. Used as separating filters, they remove particles and bacteria from diverse solutions such as buffers and therapeutic containing solutions in the pharmaceutical industry, ultrapure aqueous and organic solvent solutions in microelectronics wafer making processes, and for pre-treatment of water purification processes
Microporous membranes have a continuous porous structure that extends throughout the membrane. Workers in the field consider the range of pore widths to be from approximately 0.05 micron to approximately 10.0 microns. Such membranes can be in the form of sheets, tubes, or hollow fibers. Hollow fibers have the advantages of being able to be incorporated into separating devices at high packing densities. Packing density relates to the amount of useful filtering surface per volume of the device. Also, they may be operated with the feed contacting the inside or the outside surface, depending on which is more advantageous in the particular application.
Flat sheet membranes are typically pleated to increase the amount of membrane that can be packed into a cartridge. In commercial filter cartridges, a layer of a mesh or fabric or similar porous sheet is placed on either side of the membrane to act as a support and to provide drainage in the final cartridge. This sandwich arrangement is then pleated together. Typically, the multi-layered pleated sheet is made into a tight cylinder with the sheet ends together and with the pleats arranged axially. The sheet ends are sealed together by heat fusion or other means. Thermal fusion sealing of thermoplastic sheets such as polyethylene or polypropylene can be done directly, with no added materials. For non-thermoplastic sheets, such as PTFE, added bonding materials must be used. The pleated cylinder is placed in a cartridge housing, sometimes with a core in its inner diameter for support.
A hollow fiber porous membrane is a tubular filament comprising an outer diameter, an inner diameter, with a porous wall thickness between them. The inner diameter defines the hollow portion of the fiber and is used to carry fluid, either the feed stream to be filtered through the porous wall, or the permeate if the filtering is done from the outer surface. The inner hollow portion is sometimes called the lumen.
The outer or inner surface of a hollow fiber microporous membrane can be skinned or unskinned. A skin is a thin dense surface layer integral with the substructure of the membrane. In skinned membranes, the major portion of resistance to flow through the membrane resides in the thin skin. In microporous membranes, the surface skin contains pores leading to the continuous porous structure of the substructure. For skinned microporous membranes, the pores represent a minor fraction of the surface area. An unskinned membrane will be porous over the major portion of the surface. The porosity may be comprised of single pores or areas of porosity. Porosity here refers to surface porosity, which is defined as the ratio of surface area comprised of the pore openings to the total frontal surface area of the membrane. Microporous membranes may be classified as symmetric or asymmetric, referring to the uniformity of the pore size across the thickness of the membrane. In the case of a hollow fiber, this is the porous wall of the fiber. Symmetric membranes have essentially uniform pore size across the membrane cross-section. Asymmetric membranes have a structure in which the pore size is a function of location through the cross-section. Another manner of defining asymmetry is the ratio of pore sizes on one surface to those on the opposite surface.
The housing is usually a hollow cylinder, although other shapes are known. For ease of discussion, and not to be a limitation, cylindrical filters are discussed, although practitioners will be able to use the teachings and descriptions for other shapes. The membrane filter is located or placed within the housing. The housing serves to protect the membrane, to act as a pressure container in some cases, and to provide inlet and outlet ports or other connections for fluid flow to enter, exit, and contact the membrane filter in a controlled way.
In a practical filtration, the inlet stream is isolated from the filtered outlet stream. The filter cartridge membrane is formed and placed in the cartridge so that only one surface of the membrane contacts the inlet fluid, and the other membrane surface only contacts the filtered fluid that has passed through the membrane filter. This requires a seal to prevent the inlet fluid stream from bypassing the membrane to the outlet stream. The seal also can have provisions to allow the fluid passing through the membrane to exit the cartridge, or to serve as an inlet for fluid to be filtered to contact the membrane.
Fabricating a useful seal presents difficult problems. The seal material has to be chemically and thermally stable for the application in which the cartridge will see duty. For applications where perfluorinated membrane filters are beneficial, a sealing material of lesser properties would prevent full utility of the cartridge. The sealing material must bond well to the membrane filter, otherwise leakage can occur through the membrane-seal interface. In many cartridge designs, the seal and the cartridge housing must be bonded together liquid-tightly for the same reasons. Thermal bonding is a preferred method since it provides bonding on a molecular level, and does not require additional materials.
For hollow fiber membrane cartridges, fiber is cut or otherwise made to a specific length and a number of fibers gathered into a bundle. A portion of one or both ends of the fiber bundle are encapsulated in a material which fills the interstitial volume between fibers and forms a tube sheet. This process is sometimes called potting the fibers and the material used to pot the fibers is called the potting material. The tube sheet acts as a seal in conjunction with a filtration device. If the encapsulation process closes and seals the fiber ends, one or both ends of the potted fiber bundle are cut across the diameter or otherwise opened. In some cases, the open fiber ends are closed and sealed before encapsulation to prevent the encapsulation material from entering the open ends. If only one end is to be opened to permit fluid flow, the other end is left closed or is sealed. The filtration device supports the potted fiber bundle and provides a volume for the fluid to be filtered and its concentrate, separate from the permeating fluid. In use, a fluid stream contacts one surface and separation occurs at the surface or in the depth of the fiber wall. If the fiber outer surface is contacted, the permeating fluid and species pass through the fiber wall and are collected in the lumen and directed to the opened end or ends of the fiber. If the fiber inner surface is contacted, the fluid stream to be filtered is fed into the open end or ends and the permeating fluid and species pass through the fiber wall and are collected from the outer surface.
The pot is thermally bonded to the housing vessel in the present invention to produce a unitary end structure. The unitary end structure comprises the portion of the fiber bundle that is encompassed in a potted end, the pot and the end portion of the perfluorinated thermoplastic housing, the inner surface of which is congruent with the pot and bonded to it. By forming a unitary structure, a more robust cartridge is produced, less likely to leak or otherwise fail at the interface of the pot and the housing. The potting and bonding process is an adaptation of the method described in U.S. patent application 60/117,853, filed Jan. 29, 1999, the disclosure of which is incorporated by reference.
The cylindrical pleated filter is sealed in an analogous way. A portion of an end of the membrane and any support layers is placed in a form containing molten resin that surrounds and fills the interstitial spaces between and among the membrane and support layers. The resin containing the filter end is cooled and trimmed as needed. Several methods are availably known to those skilled in the art.
These cartridges are desirable in that they are easy to install and remove, provide protection to the membrane during installation, use and storage and make for a disposable item.
Manufacturers fabricate filter cartridges from various polymeric materials. Commonly, cartridges are made of polyolefins, polysulfone polymers, polyamides and other such well-known materials.
In the area of microelectronics, such as in the fabrication of semiconductors, such common polymeric materials cannot be used, as the conditions of production, namely highly acidic and oxidative chemicals or solvents used at high temperatures tend to dissolve or weaken most common polymeric materials. For this reason, fluorinated polymers, in particular poly(tetrafluoroethylene) (PTFE), being more chemically and thermally stable, are used. PTFE materials are the preferred materials of choice in that they are inert, capable of withstanding high temperatures and tend to have extremely low levels of extractables. However, the problems with manufacturing PTFE based cartridges are legendary. Not being thermoplastic, extreme processing parameters are required to fabricate PTFE into complex molded shapes. Additionally, PTFE materials do not tend to bond easily to any other materials including themselves.
Fluoropolymers can be placed into two general classes; those made from perfluorocarbon monomers and those made from monomers with hydrogen, chlorine, or both, and sufficient fluorine to contribute significantly to the resulting polymer properties. Perfluorinated polymers include poly(tetrafluoroethylene) (PTFE), poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), and poly(tetrafluoroethylene-co-perfluoro(alkylvinyl ether)) (PFA). The second class includes poly(ethylene-co-tetrafluoroethylene) (ETFE), poly(chlorotrifluoroethylene) (CTFE), poly(chlorotrifluoroethylene-co-ethylene) (ECTFE). Polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF) are sometimes included in the second class.
PTFE does not flow and cannot be fabricated by conventional techniques that require manipulating molten polymer. Fabricators have developed innovative processing technologies, similar to modified powder metallurgy methods, in order to utilize this polymer. FEP and PFA polymers were developed to meet the need for perfluorinated polymers that had chemical and thermal stability close to PTFE, but had the advantages of being melt processable. Plastic fabricators are able to produce a wide variety of products, such as films, extruded tubing, valves, and intricate molded parts with PFA and FEP by high-speed extrusion, injection molding and blow molding methods. PFA also has better creep resistance than PTFE, which is important for products under constant compressive or tensile load.
Polymers of the second class do not have the chemical or thermal stability of FEP and particularly of PFA types. ETFE has an upper use temperature of about 150° C. and is affected by strong oxidizing acids, organic bases and sulfonic acid at elevated temperatures. PCTFE is swollen at room temperature by some ethers and esters, halogenated solvents and toluene. PECTFE has an upper use temperature of about 163° C.-177° C., and is affected by hot amines. FEP has an upper use temperature of about 200° C. and PFA, about 260° C. Both are less affected by chemicals than the members of the second class described.
Various attempts to make chemically resistant filter cartridges have been made under the terminology of “all fluorocarbon resins,” “all fluoropolymer,” or similar terminology. These filter cartridges rely on PTFE membranes and do not teach an all perfluorinated thermoplastic cartridge.
U.S. Pat. No. 4,588,464 relates to a method for producing a filter element made wholly of a fluorocarbon resin characterized by forming sheet comprising a filter membrane made of a fluorocarbon resin superimposed on both surfaces thereof into a pleated form, bending the pleated sheet into a cylindrical form liquid-tightly welding the edge parts of the adjacent both parts of the adjacent both sides, pre-welding the pleats by heating both end parts of the cylindrical pleat-form to a temperature higher than the melting point of the net supporter, cooling the pre-welded filter, melting a thermoplastic fluorocarbon resin in a circular mold having means defining a central opening, inserting the end parts of the cooled pre-welded pleat into the molten thermoplastic fluorocarbon resin in the circular mold having a central opening to force the resin into the pleats, whereby the end part and the resin are integrally welded together, and fitting fluorocarbon caps having a prescribed shape in the end parts of the resulting filter material. This patent does not differentiate between perfluorinated thermoplastic polymers and other fluorocarbons having inferior chemical and thermal stability. Moreover, the patent is directed to the use of PTFE membranes, as the “welding the edge parts of the adjacent both parts of the adjacent both sides” requires a separate thermoplastic tape, as the PTFE membrane cannot be thermally sealed to itself, as could thermoplastic membranes.
U.S. Pat. No. 5,114,508 relates to the same invention of U.S. Pat. No. 4,588,464, without the pre-welding of the web supports to the membrane described above. As in U.S. Pat. No. 4,588,464, this patent does not differentiate between the advantages of perfluorinated thermoplastic polymers and other fluorocarbons having inferior chemical and thermal stability. The edge parts are welded with a separate tape, which would not be required for a thermoplastic membrane. No description of perfluorinated thermoplastic membranes is given.
U.S. Pat. No. 4,154,688 suggests fusing a pleated membrane cylinder to an end cap of PTFE, but states that this would be difficult, and given that PTFE is not fluid even above its melting point, PTFE would not serve as a suitable bonding agent.
U.S. Pat. No. 4,609,465 provides a filtering apparatus for removing particulates from destructive fluids. In accordance with the invention, all components of the filtering apparatus are fabricated from a fluoropolymer. These are defined as any fluorine-containing polymer, including perfluoropolymers, which are highly resistant to the deteriorative effects of destructive fluids, such as acids and/or solvents. No advantages are taught that would enable a practitioner to choose between perfluorinated thermoplastics and other fluoropolymers, such as PVDF, a preferred embodiment of the invention. PVDF is known to be soluble in aprotic solvents such as dimethylacetamide, and swollen by other solvents, such as some esters, and is therefore not suitable for uses in many applications requiring solvent resistance. Moreover, the invention of U.S. Pat. No. 4,609,465 requires a sealing ring cooperatively arranged with an end cap, with at least the surface of the sealing ring comprising a fluoropolymeric material. Such an arrangement will not provide as integral a seal under severe conditions as a thermally bonded seal.
U.S. Pat. Nos. 5,066,397 and 4,980,060 provide for hollow fiber filter elements comprising a plurality of porous hollow fiber membranes of a thermoplastic resin, each of which membranes has two end portions, at least one of said end portions of said membranes being directly fusion bonded at its periphery to form a unified terminal block in which the end portions of said membranes are fluid-tightly bonded to each other in a fused fashion. In U.S. Pat. No. 4,980,060, the membranes are fusion bonded through a thermoplastic resin medium to form a unified terminal block structure in which the end portions of said membranes are fluid-tightly bonded together in a fused fashion. It is evident that a key element in these inventions is the fusion of the individual fibers into a single end structure. Even in U.S. Pat. No. 4,980,060, the thermoplastic resin medium is only a minor fraction of the end structure as described in the disclosure. Therefore the strength of the end structure is dependent on the uniformity of the fiber-to-fiber fusion, and is dependent on the physical properties of the fiber material. Moreover, by fusing the hollow fiber membranes together, the structure of individual fibers can be compromised, with possible deleterious effects. The spaces between fibers made of polymers having high viscosity in the melt, such as perfluorinated thermoplastics, would generate bubbles during the fusion. Such bubbles would be very difficult to remove and would be sources of weakness. Therefore, a filter cartridge that had the individual fibers bonded to the end seal material would have a more uniform and stronger structure. Furthermore, these patents do not address the very serious difficulties involve in fabricating an all perfluorinated thermoplastic cartridge, which require operating at temperatures above 250° C. with high viscosity polymers. Indeed, no discussion is provide to enable a practitioner to differentiate between making filter elements from perfluorinated thermoplastics, or other fluoropolymers, which are recognized as being difficult to fabricate, and other thermoplastics, such as polysulfone or polypropylene.
U.S. Pat. No. 5,154,827 discloses a microporous polyfluorocarbon filter cartridge which uses a membrane made up of three or more sheets of aggregated microporous fluorocarbon polymer, said polymer having in the unaggregated state an individual particle diameter of not more than 0.3 micron. This process is primarily directed to the manufacture of PTFE membranes. Reduction of particle size to the range specified greatly increases the difficulty of the manufacturing process. In the present invention described in this application, membranes are made from perfluorinated thermoplastic resins reduced to approximately 100 to 1000 micron size, preferably about 300 micron size, by a suitable grinding process. Moreover, in the present invention, a single membrane sheet can be used.
U.S. Pat. No. 5,158,680 discloses a membrane-type separator having a porous film membrane consisting essentially of a layer of a porous polytetrafluoroethylene resin particle bonded structure substantially devoid of a fibrillated portion. The invention provides a method of producing a porous membrane comprising: forming a film having a hollow construction or a sheet-like construction from a polytetrafluoroethylene resin dispersion and a fiber- or film-forming polymer. The disclosure states that the “membrane” of the invention means the porous membrane obtained from the above film by removing the film-forming polymer. Such membranes required are a more complex manufacturing process and are weaker due to the particle bonded structure than those formed from phase inversion methods as described in this application, and are limited in the polymers that can be used to those that are supplied as aqueous or solvent based dispersions.
In U.S. Pat. No. 5,855,783, a pleated filter cartridge utilizes a poly(tetrafluoroethylene) paper support for poly(tetrafluoroethylene) membranes. Perfluorinated thermoplastic membranes are not contemplated or disclosed.
What is desired is a cartridge formed of a material which has the same or similar properties as PTFE resin but which is easier and less expensive to manufacture and which provides one with the capability of various modifications and complex designs which are not available with PTFE products today. The present invention provides such a device.